Few obesity prevention programs have reduced risk for weight gain over follow-up and those that have are very intensive (M duration = 52 hrs.), making dissemination difficult and costly. A brief 3-hr selective prevention program (Healthy Weight) involving participant-driven healthy dietary and physical activity lifestyle changes significantly reduced increases in BMI and obesity onset relative to alternative interventions and assessment- only controls through 3-yr follow-up. It targets young adults with body dissatisfaction as this is a prevalent risk factor for obesity and such individuals are motivated t make healthy lifestyle changes. To enhance efficacy, we added activities designed to induce dissonance regarding unhealthy dietary and activity practices, drawing from a highly efficacious dissonance-based eating disorder prevention program. Social psychology experiments show that people strive to maintain consistency between thoughts, words, and actions. Thus, we added verbal, written, and behavioral exercises wherein youth discuss costs of obesity, an unhealthy diet, and sedentary behaviors, benefits of leanness, a healthy diet, and physical activity, and institutions that contribute to obesity. Engaging in these activities putatively increases the likelihood that participants will seek to align their actions with the perspectives expressed in the sessions, resulting in healthier future lifestyle choices. A pilot trial found tha this new Project Health intervention significantly reduced increases in BMI relative to both the Healthy Weight intervention and an educational brochure condition from pre to post (d = .40 and .42 respectively), comparing favorably to effect sizes from the most efficacious intensive prevention programs. Further, other obesity prevention programs have not significantly outperformed credible alternative interventions. We propose to conduct a rigorous multisite effectiveness trial that will test whether adding the dissonance-induction elements to the originally efficacious Healthy Weight intervention improves weight gain prevention effects: 300 college students at risk for future unhealthy weight gain by virtue of body dissatisfaction will be randomized to a refined 8-hr dissonance-based Project Health intervention or an 8-hr Healthy Weight intervention, completing assessments of body fat and other outcomes at pre, post, and 6, 12, and 24 month follow-ups. Aim 1 will test whether Project Health reduces increases in % body fat relative to Healthy Weight; secondary outcomes are depressive and eating disorder symptoms. Aim 2 will test whether Project Health participants report cognitive dissonance regarding eating unhealthy foods and engaging in sedentary behaviors and whether changes in dissonance, as well as reported dietary intake and objectively measured activity level, mediate the effects of the intervention on change in % body fat. Aim 3 will investigate potential moderators that may amplify (pretest body dissatisfaction, preference for consistency) or mitigate (emotional eating, substance use) intervention effects. We will improve upon most obesity prevention trials by using a long-term follow-up, an active control condition, and direct measures of body composition and activity. PUBLIC HEALTH RELEVANCE: A brief effective obesity prevention program that can be easily and inexpensively disseminated could substantially reduce the incidence of obesity, which is vital because obesity is prevalent, causes marked morbidity and mortality, and is treatment resistant. The proposed project, which is based on promising preliminary data from rigorous prevention trials and a highly efficacious dissonance-based eating disorder prevention program, would be the first translational project to use persuasion principles from social psychology for obesity prevention.